The present invention relates to the manufacture of a planar, self-aligned emitter-base complex.
In the manufacture of hetero-bipolar transistors, the emitter-base complex is typically manufactured first. To this end, a layer sequence of different and differently doped semiconductor materials is applied on a substrate. As a rule, the uppermost part of these layers is doped for the conductivity type of the emitter and is provided with a corresponding contact. The semiconductor layer, which is doped for the formation of the base, lies somewhat deeper and must be exposed for the contacting. This can occur, for example, by local etching and re-doping of the oppositely doped layers situated thereabove.
In order to optimize the electrical properties of such an emitter-base complex, the spacing between the base and emitter must be kept as small as possible without having these two regions short-circuited. Because the geometrical dimensions are extremely small, various self-aligning manufacturing processes for hetero-bipolar transistors have been recently proposed.
In M. F. Chang et al, "GaAs/(GaAl)As Heterojunction Bipolar Transistors Using a Self-Aligned Substitutional Emitter Process", IEEE Electron Device Letters EDL-7, 8-10 (1986), a photomask layer serves for the definition of the emitter region and as a mask for the etching and subsequent doping of the base regions. An implantation step is required because the upper layers are not completely etched off down to the layer doped for the base. This doping, that is additionally introduced, is subsequently annealed and a dielectric for the formation of spacers is applied. The spacers function to apply a separate metallization of emitter and base.
In the publication Ishii et al, "High-Temperature Stable W.sub.5 Si.sub.3 /In.sub.0.53 Ga.sub.0.47 As Ohmic Contacts to GaAs for Self-Aligned HBTs", IEDM 86, pages 274-277, a self-aligning process is disclosed for manufacturing a hetero-bipolar transistor wherein the emitter metallization of WSi.sub.x serves as a mask for etching off the upper semiconductor layers doped for the conductivity type of the emitter and also for the implantation, that is also required, for doping the uppermost layers of the base region. Subsequently, this additional doping is annealed through a rapid annealing method and the emitter metallization is alloyed in. Finally, the base metallization is applied.
The publication S. Tiwari, "GaAlAs/GaAs Heterostructure Bipolar Transistors: Experiment and Theory", IEDM 86, pages 262-265 discloses a method for the manufacture of hetero-bipolar transistors on the basis of GaAs wherein the base-emitter complex is constructed self-aligning. This manufacturing process yields a planar structure. In this method, the base-emitter region is first fashioned as a mesa. The vertical sidewalls are produced from the GaAs contact layer by RIE, whereupon a silicon nitride layer is deposited on the free surface. The high-temperature stable ohmic emitter contact is based on an n.sup.+ -doped InAs layer. A tungsten silicide film is deposited on this layer in order to form the ohmic contact. With this applied emitter contact, the p.sup.+ -implantations of the base region are subsequently annealed by rapid thermal annealing at temperatures between 800.degree. C. and 900.degree. C., the length of the anneal is less than five seconds. Based on particulars of the author, a contact resistance below 2.times.10.sup.-6 ohms.times.cm.sup.2 subsequently results.